Abstract

Threaded fasteners are some of the most important machine elements. They often take on critical tasks where they are subjected to cyclical loading and are thus susceptible to fatigue damage. Typically fatigue cracks initiate in the thread root on the first loaded thread and grows in a thumbnail shape until fatigue fracture occurs. Both the Wöhlerian approach and the Fracture Mechanics approach may be used to study this phenomenon. Using the Fracture Mechanics approach requires Stress Intensity Factor (SIF) solutions, and these have been available in the literature for a long time. These solutions can be found from theoretical, experimental, and numerical methods. However, much of it is limited to opening mode (mode I) cracking only. There are several studies using numerical methods to obtain SIF for a fixed crack shape and size, some even including mode II and III; but numerical studies featuring growing cracks in threaded fasteners are sparse. Using numerical tools to model fatigue crack growth offers a highly useful capability, as experimental procedures can be very time consuming, allowing testing and design processes to be greatly accelerated with the possibility of crack growth modelling added to the toolbox. This work investigates crack growth in a threaded fastener using numerical methods. The software used is BEASY, which utilises the Boundary Element Method (BEM) and provide analysis tools which specialise in fatigue analysis. In the analyses the BEM models using surface mesh rather than 3D elements to model the continuum, which greatly reduces the number of elements needed for the model and lends itself well to coping with geometrical irregularities which may be present in a fatigue crack. The bolt is loaded in axial tension. Cracks are inserted into the root of the first loaded thread and cyclic loading is applied to induce growth. The growth is dominated by the opening mode, but mode II and mode III are also present. It is of interest to study how the crack develops; in particular its shape, critical size, and the influence of mode II and III over the course of the crack growth. The results are validated using data from literature.